Not sure that needs a visual to understand, although it would be interesting to to see how you would present it, in my mind it's just another form of bottle-neck. In essence:
The lead bus has to stop for longer to pick up waiting passengers than the following bus. The less time the following bus has to stop, the closer it gets to the bus in front, so the less time for new passengers to accumulate. And so on down the chain.
The lead bus has to stop for longer to pick up waiting passengers than the following bus. The less time the following bus has to stop, the closer it gets to the bus in front, so the less time for new passengers to accumulate. And so on down the chain.